1. Field of the Invention
This invention relates to a roll stamper used to continuously manufacture a substrate for a high-density information recording medium such as an optical disk or optical card by transferring a pre-format pattern to a substrate surface, and to an apparatus for forming the high-density information recording medium substrate by using the stamper.
2. Related Background Art
For example, an injection molding method, a compression molding method and an extrusion molding method are known as methods for manufacturing the substrate for a high-density optical recording medium such as an optical disk or optical card.
When the injection molding method or compression method is used, strict conditions are set with respect to the temperature and the pressure of a molten resin and the precision of dies and so on to achieve the desired flatness and smoothness of the substrate, to limit a warp of the substrate and to prevent inclusion of air bubbles in the molten resin, and a large-scale molding machine is therefore required, resulting in an increase in manufacturing cost. In the case of injection molding, it is difficult to manufacture a substrate having a small thickness of 0.4 mm or less and a small double refraction index. In the case of compression molding and injection molding, the productivity is low because molded sheets are separately formed one after another.
An information recording medium substrate may be manufactured by extrusion molding in such a manner that a sheet of a molten resin extruded from an extruder is pressed by a roll stamper to form the substrate while transferring a pre-format pattern for providing a pre-format signal, a tracking signal and the like. Such an extrusion molding process is therefore advantageous in terms of mass production of a substrate and has recently been studied as a method for forming an information recording medium substrate.
Roll stampers to be used to form an information recording medium substrate by extrusion molding as described above have conventionally been manufactured by the following methods:
(1) a method in which a pre-format pattern is directly formed in a circumferential surface of a roll base having the circumferential surface polished to form a specular surface (hereinafter referred to as "specular roll base");
(2) a method in which a flexible stamper having a pre-format pattern previously formed on its surface is uniformly bonded to a specular circumferential surface of a specular roll with an adhesive; and
(3) a method in which a flexible stamper having a pre-format pattern previously formed on its surface is fixed to a specular circumferential surface of a specular roll with screws.
In the case of method (1), if a defect is caused in only one portion of the pre-format pattern, the whole roll stamper must be exchanged, and the molding efficiency is therefore low. In the roll stamper manufactured by method (2), the adhesive is changed in quality with time by the heat of the extruded resin sheet or the heat of the roll so that the bonding force is reduced and the stamper may easily come off the specular roll. Therefore, the life of the roll stamper is very short. Moreover, air bubbles are liable to be generated in the adhesive. Unnecessary irregularities in the stamper surface caused by the air bubbles are transferred to the substrate, so that the pattern may not be formed accurately and there is a problem in resulting transfer failure. In the case of the roll stamper manufactured by method (3), it is difficult to limit the gap between the flexible stamper and the roll base, and therefore non-uniformity in the temperature on the stamper surface tends to occur, thereby resulting in molding failure. A method is therefore needed which enables a flexible stamper to be securely fixed on a specular roll base so that the stamper surface (reverse surface) on which no pre-format pattern is formed is brought into close contact with the circumferential surface of the specular roll base while occurrence of a strain in the stamper is prevented.